Because she is still very much alive when placed on the pyre, the experience is far from painless. Widows, suddenly feeling the lick of fire on their skin or on their hair, might try to save themselves - to the disgusted boos of the grieving relatives.
They might shove her back to the pyre with sticks or anything else handy. Unsuccessful, they might have to disable her ability to move by breaking her legs and tossing her back to the side of her husband's corpse.
The interesting fact about this custom is that this action on the part of the widow is voluntary - after being indoctrinated all her life what a glorious thing it is to remain so loving and fathful to her husband that she even chooses to die once he dies.
There are - or have been - a lot of customs which haven't exactly been kind to women. Those who uphold them always point out that women themselves choose to uphold these customs. And, of course, they do.
How can they choose an alternative, when they only know the custom the land allows them. It's a choice, with no choice.